Automatic volume control circuit



June 20, 1939. R. A. WEAGANT AUTOMATIC VOLUME CONTROL CIRCUIT Filed Jan.25, 1958 H i v P 3 JH fl .u\: m m & ESEQ N m m ,T m h m. h u m mm. i w V.w n m. H\ u H H u H a mm ,O -O m m W M mm m W f N m H H H H i mfipfi tu n H H H v F1 T H. A m B K O A T ktv WQ .QQSEKQ INVENTOR. ROYA WEAGANT7 414 ATTORNEY.

Patented June 20, 1939 UNKTED STATES PATENT OFFICE AUTOMATIC VOLUlVIECONTROL CIRCUIT Application January 25, 1938, Serial No. 186,792

5 Claims.

My present invention relates to automatic volume control (AVC) circuits,and more particularly to an AVC circuit adapted to regulate bothamplification and detection with the electrodes 5 of a triode.

One of the main objects of my present invention is to utilize thecathode, grid and plate of a triode to provide a detector and an AVCrectifier; the rectifier functioning to decrease the radio 10 amplifiergain when the signals increase in amplitude above a desired level, andthe detection efficiency being concurrently decreased.

Still other objects of my invention are to improvegenerally thesimplicity and efliciency of 5 receivers provided with AVC circuits; andmore especially to provide an AVC arrangement which is economicallymanufactured and assembled in aradio receiver.

The novel features which I believe to be char- 2 acteristic of myinvention are set forth in particularity in the appended claims; theinvention itself, however, as to both its organization and method ofoperation will best be understood by reference to the followingdescription taken in connection with the drawing in which I haveindicated diagrammatically a circuit organization whereby my inventionmay be carried into effect.

Referring now to the drawing, there is schematically shown a receiver ofconventional con- 30 struction embodying my invention. The signalcollector I, which can be a grounded antenna circuit, is coupled to thetunable input circuit 2 of the radio frequency amplifier tube 3. The

cathode of tube 3 is at ground potential; the,

plate of the tube is connected to a proper positive potential point B onthe customary voltage supply bleeder resistor P. An intermediate point Gof the latter is at ground potential. The plate of tube 3 is coupled tothe tunable input circuit 4; the latter is disposed between the inputelectrodes of the second radio amplifier tube 5. The cathode of thelatter is at ground potential; the plate circuit of tube 5 may includethe tunable network 6. The dotted line 1 denotes the customarymechanical coupling between the rotors of the variable condensers usedin tunable circuits 2, 4, 6. The amplifiers 3 and 5 can denote the usualnetworks of a superheterodyne receiver, if desired, prior to the sec- 50nd detector. The tubes 3 and can be screen grid or pentode tubes.

The receiver detector comprises a triode 8; the cathode 9 is connectedto a point C2 on P by a resistor R1. The control grid I0 is connected 55to a desired point on resistor R1 through a path including coil ll,resistor R and adjustable tap E2. The coil H is reactively coupled tothe out-- put circuit of amplifier 5; a radio frequency bypass condenserI3 is connected. between the.- cathode and the grid end of resistor R.Audio 5 voltage developed across the load resistor R is impressed on theaudio amplifier [4; the audio coupling condenser I5 is used to couplethe grid of tube Hi to the load resistor R. The usual leak resistor I6may connect the grid of tube 14 to a negative point C1 on P. The outputof tube M may be further amplified, and finally reproduced inaloudspeaker.

The plate I! of tube 8 is connected to the cathode through a pathincluding load resistor 15 R2, shunted by a radio frequency by-passcondenser I 8, coil l 9 and condenser 20. The coil I9 is reactivelycoupled to circuit 6. The adjustable tap 2| connects the junction ofcoil l9 and condenser to a negative point C on P. The po- 20 tentialdifference between points C and C2 on P is designated by the referenceletter V. The lead 22, designated as the AVG connectiomconnects the lowpotential end of each input circuit 2 and 4 to the anode end of resistorR2. The direct current blocking condensers 23 and 24 connect the lowpotential ends of each of circuits 2 and 4 to the grounded cathodes.Resistors 25 are included in the lead 22 to suppress the pulsatingvoltage components of rectified voltage developed across resistor R2.

The operation of the tube 8 and its associated networks will now beexplained. It should be understood that the receiver can be oneoperating in the broadcast range, say 500 to 1500 k. c. The signalvoltage developed across circuit 6 is impressed on each of coils l9 andl l. The detection action by the circuit included between grid l0 andcathode 9 provides the audio voltage; the rectification by the circuitbetween anode l1 and cathode 9 provides the direct current voltageacross R2 used for AVG. The normal bias for the amplifier grids is thevoltage between ground and point C on bleeder P. That is to say, when nosignals are received the amplifier tubes 3 and 5 are operating atmaximum gain; the negative potential not being developed across resistorR2. In this no-signal condition there is no plate or grid current flowin tube 8.

When weak signals are received, and the ampli- 5o tude of which signalsis less than the magnitude of the delay bias V, no plate current flowsintube 8. If grid current flows, however, it tends to render thedetector cathode ,9 more positive than before, thus adding a little moredelay voltage to the bias v. However, it should be noted that this extradelay voltage is not added to the bias applied to the amplifier grids.There is no delay bias applied to the grid l0, hence the signaldetection is normal. Therefore, it will be seen that for weak signalreception audio voltage is fed to the amplifier I4, while the AVGrectification is delayed.

Upon the received signal carrier amplitude exceeding the delay voltageapplied to plate l1, current flows through the resistor R1, as well asthrough resistor R2. The direct current voltage developed acrossresistor R2 is employed to increase the negative bias on the signalgrids of amplifier tubes 3 and 5, and it will be noted that the negativevoltage applied through lead 22 is added to the voltage V. Concurrently,the grid I0 is biased negatively with respect to the cathode 9, sincethe grid I0 is connected through resistor R and tap I2 to anintermediate point on resistor R1. This biasing of grid I 0 reduces thedetection efiiciency of the rectifier comprising grid l0 and cathode 9and the associated circuit.

By a suitable choice of a magnitude of resistor R1, and suitablepositioning of tap l2 on the latter, various efiects may be produced.With the tap l2 positioned adjacent the cathode end of resistor R1, thegrid circuit acts as a simple diode, while the plate circuit produces anadditional amplifier bias that increases linearly with the excess ofsignal voltage over the bias V. On the other hand, with the tap l2adjusted to the extreme right end of the resistor R1, the plate currentthrough the resistor may bias the grid l0 so that no rectification takesplace in the grid circuit beyond a certain signal strength dependentupon the ratio of signal voltages applied to the coils II and I9. Thiscan provide a silencing action for strong crashes of interference.

With this type of AVG circuit the automatic decrease of the amplifiergain will cooperate with the detector efliciency decrease so that themagnitude of the audio voltage delivered to the amplifier I4 ismaintained substantially constant in spite of fading of the receivedcarrier signal. A system of this type is of a special utility inconnection with receivers of limited amplifier tubes, and where it isdesired to have an efficient AVC system. It will be'particularly notedthat the AVG action is secured with an ordinary triode tube, and thatthe AVG action itself is of the delayed type.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modifications may be made without departingfrom the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a tube of the type including a cathode, controlgrid and plate, a signal input circuit connected between the cathode andcontrol grid to provide a diode detection circuit, said signal inputcircuit including imped ance means for developing an audio currentvolt-' age from impressed signals, a second signal input circuitconnected between the plate and cathode, said second circuit includingimpedance means for developing a direct current voltage from im pressedsignals, and impedance means, other than said two impedance means,common to said two input circuits for automatically controlling thedetection eificiency of said cathode and control grid in response tocurrent flow between said cathode and plate.

2. In a system as defined in claim 1, a source of direct currentvoltage, and means connecting said source between said cathode and platein a manner such that the plate is biased to a predetermined negativevoltage with respect to the cathode thereby to delay the development ofsaid direct current voltage.

3. In a system as defined in claim 1, said common means comprising aresistor in said second circuit, and means providing a direct currentvoltage connection between said control grid and a point on saidresistor such that the grid becomes increasingly negative with respectto the cathode as said current flows.

4. In combination with a source of modulated carrier signals, anelectron discharge tube of the v type including at least a cathode,control grid and plate, a signal circuit coupling said source to saidcathode and plate, said signal circuit including a load resistor fordeveloping a voltage from received signals, an impedance elementdisposed in the space current path between said cathode and plate, asecond signal circuit coupled to said source and. connecting the gridand cathode to provide a'diode detector circuit, and a direct currentvoltage connection between said control grid and a point on saidimpedance such that space current flow through the latter results innegative biasing of the control grid.

5. In combination with a source of modulated carrier signals, anelectron discharge tube of the a type including at least a cathode,control grid and plate, a signal circuit coupling said source to saidcathode and plate, said signal circuit including a load resistor fordeveloping a voltage from received signals, an impedance elementdisposed in the space current path between said cathode and plate, asecond signal circuit coupled to said source and connecting the grid andcathode to provide a diode detector circuit, a

direct current voltage connection between said control grid and a pointon said impedance such that space current flow through the latterresults in negative biasing of the control grid, said source including asignal amplifier tube, and a gain control connection between said loadresistor and said amplifier tube.

ROY A. WEAGANT.

